Irritable bowel syndrome (IBS) is a common disorder in children that can interfere with the quality of life and daily activities in many patients. IBS is associated with alteration in gut flora. Evidence for the role of enteric bacteria is seen in several studies documenting the new onset of IBS following bacteriologically confirmed bacterial gastroenteritis and other studies demonstrating evidence of low-grade mucosal inflammation and immune activation in patients with IBS. The enteric flora has also been implicated in the pathogenesis since patients with IBS may harbor bacterial overgrowth with amelioration of symptoms upon eradication. In addition, a significant body of research supports a role for inflammatory and immune processes, in contributing to enteric neuromuscular dysfunction. Probiotics can induce changes in the enteric flora directly through the augmentation of commensal bacteria or indirectly through a reduction in pathogen or non-pathogen related inflammation to favorably influence gut function. Adult studies show efficacy of a promising probiotic agent, VSL#3. We hypothesize that VSL#3 is effective in treating symptoms of irritable bowel syndrome in children. The goal of this study is to determine whether the oral administration of the probiotic VSL#3 under randomized, placebo-controlled conditions will improve symptoms of IBS in children. The specific aims of the study are as follows: (1)Determine the efficacy of different doses of VSL#3 in the treatment of children with diarrhea-predominant irritable bowel syndrome: VSL#3 or placebo will be blindly given to the children at a dose of 900, 450, or 90 billion cells daily for eight weeks. Response to therapy will be recorded and collected on weekly basis using a 7-point Likert. (2)Study the gut colonization of VSL#3 in children with IBS: Stool recovery of VSL#3 will be performed before, during and at the end of consumption of the study agent. (3)Determine safety of VSL#3 use in children, by closely monitoring for any potential adverse events. Description for public health: Irritable bowel syndrome is a disease that affects many children and interferes with daily living. Studies suggest that this disorder is related to the intestinal bacteria. We propose using "good" bacteria to help children with this disorder.